There is a need for an environmentally biodegradable packaging thermoplastic as an answer to the tremendous amounts of discarded plastic packaging materials. U.S. plastic sales in 1987 were 53.7 billion pounds of which 12.7 billion pounds were listed as plastics in packaging. A significant amount of this plastic is discarded and becomes a plastic pollutant that is a blot on the landscape and a threat to marine life. Mortality estimates range as high as 1-2 million seabirds and 100,000 marine mammals per year.
A further problem with the disposal of plastic packaging is the concern for dwindling landfill space. It has been estimated that most major cities will have used up available landfills for solid waste disposal by the early 1990's. Plastics comprise approximately 3 percent by weight and 6 percent of the volume of solid waste.
One other disadvantage of conventional plastics is that they are ultimately derived from petroleum, which leaves plastics dependent on the uncertainties of foreign crude oil imports. A better feedstock would be one that derives from renewable, domestic resources.
However, there are good reasons for the use of packaging plastics. They provide appealing aesthetic qualities in the form of attractive packages which can be quickly fabricated and filled with specified units of products. The packages maintain cleanliness, storage stability, and desirable qualities such as transparency for inspection of contents. These packages are known for their low cost of production and chemical stability. This stability, however leads to very long life of plastic, so that when its one time use is completed, discarded packages remain on, and in, the environment for incalculably long times.
The polymers and copolymers of lactic acid have been known for some time as unique materials since they are biodegradable, biocompatible and thermoplastic. These polymers are well behaved thermoplastics, and are 100 percent biodegradable in an animal body via hydrolysis over a time period of several months to a year. In a wet environment they begin to show degradation after several weeks and disappear in about a year's time when left on or in the soil or seawater. The degradation products are lactic acid, carbon dioxide and water, all of which are harmless.
In practice, lactic acid is converted to its cyclic dimer, lactide, which becomes the monomer for polymerization. Lactic acid is potentially available from inexpensive feedstocks such as cornstarch or corn syrup, by fermentation, or from petrochemical feedstocks such as ethylene. Lactide monomer is conveniently converted to resin by a catalyzed, melt polymerization, a general process well-known to plastics producers. By performing the polymerization from an intermediate monomer, versatility in the resin composition is permitted. Molecular weight can be easily controlled. Compositions can be varied to introduce specific properties.
Homopolymers and copolymers of various cyclic esters such as glycolide, lactide, and the lactones have been disclosed in numerous patents and scientific publications. Early patents disclosed processes for polymerizing lactic acid, lactide, or both, but did not achieve high molecular weight polymers with good physical properties, and the polymer products were frequently tacky, sticky materials. See, for example, U.S. Pat. Nos. 1,995,970; 2,362,511; 2,683,136; and 3,565,869. The Lowe patent, U.S. Pat. No. 2,668,162, teaches the use of pure glycolide and lactide to achieve high molecular weight polymers and copolymers of lactide. Copolymerization of lactide and glycolide imparted toughness and improved thermoplastic processability as compared to the homopolymers. Emphasis was placed on orientable, cold-drawable fibers. Films are described as self-supporting, or stiff, tough, and either clear or opaque. The polymers were high melting and stiff. U.S. Pat. No. 3,565,869 discloses the typical attitude to the presence of monomer in polyglycolide--the removal of the monomer from the product. In U.S. Pat. No. 2,396,994, Filachione et al disclose a process for producing poly(lactic acids) of low molecular weights from lactic acid in the presence of a strong mineral acid catalyst. In U.S. Pat. No. 2,438,208, Filachione et al disclose a continuous process for preparing poly(lactic acid) with an acidic esterification catalyst. In U.S. Pat. No. 4.683,288, Tanaka et al disclose the polymerization or copolymerization of lactic and/or glycolic acid with a catalyst of acid clay or activated clay. The average molecular weight of the polymer is at least 5,000 and preferably 5,000-30,000. In U.S. Pat. No. 4,789,726, Hutchinson discloses a process for production of polylactides or poly (lactide-co-glycolide) of specified low-medium molecular weight, by controlled hydrolysis of a higher molecular weight polyester.
Similar disclosures in the patent and other literature developed the processes of polymerization and copolymerization of lactide to produce very strong, crystalline, orientable, stiff polymers which were fabricated into fibers and prosthetic devices that were biodegradable and biocompatible, sometimes called absorbable. The polymers slowly disappeared by hydrolysis. See, for example, U.S. Pat. Nos. 2,703,316; 2,758,987; 3,297,033; 3,463,158; 3,498,957; 3,531,561; 3,620,218; 3,636,956; 3,736,646; 3,797,499; 3,839,297; 3,982,543; 4,243,775; 4,438,253; 4,496,446; 4,621,638; European Patent Application EP0146398, International Application WO 86/00533, and West German Offenlegungsschrift DE 2118127 (1971). U.S. Pat. Nos. 4,539,981 and 4,550,449 to Tunc teach high molecular weight materials suitable for prosthetic devices, while in EP 321,176 (1989) Tunc discloses a process for orienting resorbable thermoplastic members made from polylactides disclosed in the U.S. patents. U.S. Pat. No. 4.603,695 discloses sheet surgical adhesion preventatives. U.S. Pat. No. 4,534,349 discloses molded medical devices for nerve repair. R. G. Sinclair et al in, Preparation and Evaluation of Glycolic and Lactic Acid-Based for Implant Devices Used in Management of Maxillofacial Trauma, I; AD748410. National Technical Information Service, prepares and evaluates polymers and copolymers of L-lactide and glycolide, the polymers were light brown in the case of the polyglycolide with increasing color in the case of the polymers incorporating more lactide, in a second series of polymers the homopolymer of lactide was a snow white crystalline solid.
Other patents teach the use of these polymers as stiff surgical elements for biomedical fasteners, screws, nails, pins, and bone plates. See, for example, U.S. Pat. Nos. 3,739,773; 4,060,089; and 4,279.249.
Controlled release devices, using mixtures of bioactive substances with the polymers and copolymers of lactide and/or glycolide, have been disclosed. See, for example, U.S. Pat. Nos. 3,773,919; 3,887,699; 4,273,920; 4,419,340; 4,471,077; 4,578,384; in 4,728,721, Yamamoto et al disclose the treatment of biodegradable high molecular weight polymers with water or a mixture of water and water soluble organic solvents so as to remove unreacted monomer or monomers and polymers of low polymerization degree. Poly(lactic acid) and copolymers of lactic and glycolic acid of 2,000 to 50,000 molecular weight are prepared by direct condensation for use as an excipient for microcapsules; R. G. Sinclair, in Environmental Science & Technology, 7 (10), 955 (1973). R. G. Sinclair, Proceedings, 5th International Symposium on Controlled Release of Bioactive Materials, 5.12 and 8.2, University of Akron Press, 1978. These applications of lactide polymers and copolymers required tough, or glassy materials, that were grindable and did not disclose physical properties for obvious use in thermoplastic packaging materials. R. G. Sinclair in, Lactic Acid Polymers--Controlled Release Applications for Biomedical Use and Pesticide Delivery; Proc. of the First Annual Corn Util. Conf., p. 211, Jun. 11-12, 1987, discusses some of the advantages of lactides as homopolymers and as copolymers with glycolide and caprolactones.
Some mention has been disclosed in the prior art for use of lactide copolymers for packaging applications. Thus, in the aforementioned patent to Lowe, clear, self-supporting films are noted of a copolymer of lactide and glycolide. In U.S. Pat. No. 2,703,316 lactide polymers are described as film formers, which are tough and orientable. "Wrapping tissue" was disclosed that was tough, flexible, and strong, or pliable. However, to obtain pliability the polylactide must be wet with volatile solvent, otherwise, stiff and brittle polymers were obtained. This is an example of the prior art which teaches special modifications of lactide polymers to obtain pliability. Thus, in U.S. Pat. No. 3,021,309, lactides are copolymerized with delta valerolactone and caprolactone to modify lactide polymers and obtain tough, white, crystalline solids. Soft, solid copolymer compositions are mentioned only with the copolymer of caprolactone and 2,4-dimethyl-4-methoxymethyl-5-hydroxypentanoic acid lactone, not with lactide compositions. U.S. Pat. No. 3,284,417 relates to the production of polyesters which are useful as plasticizers and intermediates for the preparation of elastomers and foams. This patent excludes lactides and uses compositions based on 7 to 9 membered ring lactones, such as epsilon caprolactone, to obtain the desired intermediates. No tensile strength, modulus, or percent elongation data are given. U.S. Pat. No. 3,297,033 teaches the use of glycolide and glycolide-lactide copolymers to prepare opaque materials, orientable into fibers suitable for sutures. It is stated that "plasticizers interfere with crystallinity, but are useful for sponge and films". Obvious in these disclosures is that the lactide polymers and copolymers are stiff unless plasticized. This is true also of U.S. Pat. No. 3.736,646, where lactide-glycolide copolymers are softened by the use of solvents such as methylene chloride, xylene, or toluene. In U.S. Pat. No. 3,797,499 copolymers of L-lactide and D,L-lactide are cited as possessing greater flexibility in drawn fibers for absorbable sutures. These fibers have strengths greater than 50.000 psi with elongation percentages of approximately 20 percent. Moduli are about one million psi. These are still quite stiff compositions compared to most flexible packaging compositions, reflecting their use for sutures. U.S. Pat. No. 3,844,987 discloses the use of graft and blends of biodegradable polymers with naturally occurring biodegradable products, such as cellulosic materials, soya bean powder, rice hulls, and brewer's yeast, for articles of manufacture such as a container to hold a medium to germinate and grow seeds or seedlings. These articles of manufacture are not suitable for packaging applications.
U.S. Pat. No. 4,620,999 discloses a biodegradable, disposable bag composition comprised of polymers of3-hydroxybutyrate and 3-hydroxybutyrate/3-hydroxyvalerate copolymer. Lactic acid, by comparison, is 2-hydroxy propionic acid. U.S. Pat. No. 3,982,543 teaches the use of volatile solvents as plasticizers with lactide copolymers to obtain pliability. U.S. Pat. Nos. 4,045,418 and 4,057,537 rely on copolymerization of caprolactone with lactides, either L-lactide, or D,L-lactide, to obtain pliability. U.S. Pat. No. 4,052,988 teaches the use of poly (p-dioxanone) to obtain improved knot tying and knot security for absorbable sutures. U.S. Pat. Nos. 4,387,769 and 4,526,695 disclose the use of lactide and glycolide polymers and copolymers that are deformable, but only at elevated temperatures. European Patent Application 0108933 using a modification of glycolide copolymers with polyethylene glycol to obtain triblock copolymers which are taught as suture materials. As mentioned previously, there is a strong consensus that pliability is obtained in lactide polymers only by plasticizers which are fugitive, volatile solvents, or other comonomer materials.
Copolymers of L-lactide and D,L-lactide are known from the prior art, but citations note that pliability is not an intrinsic physical property. The homopolymers of L-lactide and D,L-lactide, as well as the 75/25, 50/50, and 25/75, weight ratio, of L-/D,L-lactide copolymers are exampled in U.S. Pat. No. 2,951,828. The copolymers have softening points of 110.degree.-135.degree. C. No other physical property data are given relating to stiffness and flexibility. The 95/5, 92.5/7.5, 90/10, and 85/15, weight ratio, of L-lactide/D,L-lactide copolymers are cited in U.S. Pat. Nos. 3,636,956 and 3,797,499. They are evaluated as filaments from drawn fibers and have tensile strengths in excess of 50,000 psi, moduli of about one million psi, and percent elongations of approximately 20 percent. Plasticizers, the same as in U.S. Pat. No. 3,636,956, above, were used to impart pliability. A snow-white, obviously crystalline polymer, is cited in Offenlegungsschrift 2118127 for a 90/10, L-lactide/D,L-lactide copolymer. No physical properties were given for this copolymer. The patent teaches the use of surgical elements.
Canadian Patent 808,731 cites the copolymers of L- and D,L-lactide where a divalent metal of Group II is part of the structure. The 90/10, L-/D,L-lactide copolymer (Example 2) and the L-lactide homopolymer were described as "suitable for films and fibers". The 90/10 copolymer is described as a snow-white copolymer and the homopolymer of L-lactide can be molded to transparent films. (The more crystalline polymer should be the opaque, or snow-white material, which is the homopolymer.) The patent discloses "the fact that the novel polylactides of the present invention contain the metallic component of the catalyst in the form of a lactate is believed to be of significance". Furthermore, "the polylactides find utility in the manufacture of films and fibers which are prepared by conventional thermoplastic resin manufacturing methods". No physical property data are given on the strength and flexibility of the films.
Canadian Patent 863,673 discloses compositions of L-lactide and D,L-lactide copolymers in the ratios of 97/3, 95/5, 92.5/7.5, 90/10, and 85/15 ratios of L-/D,L-lactide, respectively. These were all characterized as drawn filaments for surgical applications. Tensile strength, approximately 100,000 psi, was high, elongation was approximately 20 percent and plasticizers were mentioned to achieve pliability. D,L-lactide compositions of less than 15 weight percent are claimed.
Canadian Patent 923,245 discloses the copolymers of L- and D,L-lactide (Example 15). The 90/10 copolymer is described as a snow white polylactide. The polylactides prepared by the methods of the patent are stated to have utility in the manufacture of films or fibers prepared by conventional thermoplastic resin fabricating methods.
U.S. Pat. No. 4,719,246 teaches the use of simple blending of poly L-and poly (D-lactide), referred to as poly (S-lactide) and poly (R-lactide). The examples are all physical mixtures. The special properties of the "interlocking" stem from racemic compound formation (cf. "Stereochemistry of Carbon Compounds", E. L. Eliel, McGraw-Hill, 1962, p. 45). Racemic compounds consist of interlocked enantiomers, that is, the D and L forms (or R and S) are bonded to each other by polar forces. This can cause a lowering, or raising, of the crystalline melting points, depending on whether the D to D (or L to L) forces are less, or greater, than the D to L forces. Required of polymer racemic compounds to enhance the effect (and stated in U.S. Pat. No. 4,719,246, Column 4, line 48) are homopolymers, or long chain lengths, of both D and L. The great symmetry or regularity of these structures permit them to fit together, or interlock, by very regular polar forces, either because they are the same, or mirror images. This leads to considerable crystallinity. The art of racemic compounds has a long history that goes back to classical chemistry.
Additional related art includes: Low molecular weight poly D,L-lactide has been recently added to high molecular weight D,L-lactide along with a drug such as caffeine, salicylic acid, or quinidine, see R. Bodmeier et al, International J. of Pharm. 51, pp. 1-8, (1989). Chabot et al in polymerizing L-lactide and racemic D,L-lactide for medical applications removed residual monomer and lower oligomers, see Polymer, Vol. 24, pp. 53-59, (1983). A. S. Chawla and Chang produced four different molecular weight D,L-lactide polymers but removed monomer for in vivo degradation studies, see Biomat., Med. Dev. Art. Org., 13(3&4), pp. 153-162, (1985-86). Kleine and Kleine produce several low residual monomer, poly(lactic acids) from D,L-lactide while determining lactide levels during the polymerization, see Macromolekulare Chemie, Vol. 30, pp. 23-38, (1959); Kohn et al also makes a low residual monomer product while monitoring the monomer content over time, see Journ. Appl. Polymer Science, Vol. 29, pp. 4265-4277, (1984). M. Vert et al teaches high molecular weight polylactides with elimination of residual monomer, see Makromol. Chem., Suppl. 5, pp. 30-41, (1981). M. Vert, in Macromol. Chem., Macromol. Symp. 6, pp.109-122, (1986), discloses similar poly(L-/D,L-lactide) polylactides, see Table 6, p. 118. In EP 311,065 (1989) poly D,L-lactide is prepared as an implant material for drug delivery as the material degrades, the material contains drugs, low molecular weight polylactide, and other additives; EP 314,245 (1989) teaches a polylactide having a low amount of residual monomer, the polymer is prepared by polymerization of meso D,L-lactide or other monomers; West German Offenlegungsschrift DE 3,820,299 (1988) teaches the polymerization of meso D,L-lactide with lactides, however, the advantages of the present invention are not obtained.
Nowhere in the prior art is it disclosed that lactic acid or lactide polymers, can be the source of pliable, highly-extensible compositions by the use of lactide monomers, or lactic acid, or oligomers of lactic acid, or derivatives of oligomers of lactic acid, or oligomers of lactide as the plasticizer. None of the prior compositions are suitable for well-defined packaging needs.